It is hypothesized that: (a) several discrepancies in digoxin pharmacokinetics, (b) part of the intersubject variability in digoxin serum or plasma concentrations, and (3) part of the problem in separating toxic and nontoxic patients taking digoxin on the basis of serum digoxin concentrations may be the consequence of the non-specificity of the radioimmunoassay (RIA) methods, i.e. measurement of not only unchanged digoxin but also digoxin metabolites by the RIA. A proposed study in normal volunteers and another study in adult cardiac patients taking digoxin chronically will test these hypotheses. A third study will allow measurement of both plasma concentrations and concentrations in heart tissue in children 6 months to 15 years of age, where the biological samples are taken during routine procedures in open heart surgery; such measurements have been made previously in infants and adults by means of non-specific assay methods, but not in children in the age range indicated, and, in patients of any age by a specific method of assay. Biological specimens collected in the three human studies will be assayed by the non-specific RIA method and a specific method in which digoxin is first separated from its metabolites by a high-performance liquid chromatography (HPLC) method then the digoxin is measured in part of the effluent from the column by the RIA method.